(a) Field of the Invention
The present invention relates to a process for the nitration of aromatic compounds, and more particularly to a process for the nitration of aromatic compounds through a nitration process which does not use mixed acids or nitric acid as the nitrating agent.
(b) Description of the Prior Art
Aromatic nitro compounds produced through a nitration process are widely used as starting materials for the production of, or the intermediate products for, a variety of agro-chemicals, pharmaceuticals, dyes, explosives, and rubber chemicals. Among the different aromatic nitro compounds, nitrobenzene and dinitrotoluene are of great importance as starting materials for MDI(Methylene Diphenyl Isocyanate) and TDI(Toluene Diisocyanate), respectively.
A mixed acid containing concentrated nitric acid and concentrated sulfuric acid is presently used in the nitration of aromatic compounds (see Euler, H. Ann. Chem., 330, 280(1903)). When such a mixed acid is used in the nitration process, a large amount of waste sulfuric acid is produced as a by-product, of which treatment is a burden to the industry. Further, the resulting waste water, including the water used for the cleaning process, is a dire environmental problem that needs to be solved.
In an effort to solve these problems, it is reported that Olin corporation of USA has recently developed a nitration process that can effectively convert toluene into dinitrotoluene using concentrated nitric acid only but not using sulfuric acid (see Quakenbush A. B. and Pennington B. T., ACS Symposium Series 623, American Chemical Soc., Washington. D.C. 1996, pp 214-222). However, a separate process is needed in this method to maintain a high concentration of nitric acid in order to prevent a reduction in reactivity by the water produced as a by-product during the nitration process. Also, as it is not easy to control the reactivity of the nitration process, additional trinitrotoluene is produced by the continuing reaction. Moreover, the problem of waste water remains, and the separation of the final products after the completion of the reaction is not an easy task.
Although another nitration process has been developed in which dinitrogen pentoxide in an organic solvent is used, it is difficult to apply the process for commercial purposes because an efficient process for preparing dinitrogen pentoxide has yet to be established. (see Millar R. W. et al., ACS Symposium Series 623, American Chemical Soc., Washington. D.C., 1996, pp 104-121; Millar R. W. et al., U.S. Pat. No, 4,820,859 (1989)).
Suzuki et al. disclosed a nitration process which can be easily carried out in an organic solvent by using gaseous nitrogen dioxide and an oxidant, ozone(see Suzuki H., et al., Chem. Letters, 1421(1993); Suzuki H., et al., J.C.S. Chem Commun, 1409(1991); EPO 497 989 A1, 1991). This process can solve the above stated problem occurring in the conventional processes using mixed acid. Further, Korean Patent Application No. 95-32526 filed by the inventor of the present patent application discloses a process which can effectively carry out a nitration process by using nitrogen dioxide and ozone in the presence of nitric acid. However, since ozone used as an oxidant during the nitration reaction can only be obtained by discharging oxygen at a high voltage, an expensive ozone generator and large amounts of electricity are needed for the mass production of ozone. Accordingly, this process has feasibility problems with regard to an industrial application.
Further, Suzuki et al. also discloses a nitration process which uses dichloroethane(EDC, ethylene dichloride) as a solvent and use no ozone but oxygen in the presence of catalysts such as Fe(acac).sub.3 (see Suzuki H., et al., to J.C.S. Perkin Trans., 1, 2358(1996)). However, a long reaction time (e.g. 12 to 36 hours) is needed in this process even though large amounts of nitrogen dioxide (31 equivalent) and a catalyst (10 mol %) are used. Further, if the reaction is conducted without the use of an additional solvent, the reaction is slow. Thus, the nitration process using nitrogen dioxide and oxygen has low reactivity and can not be a substitute for the conventional commercial nitration processes. Akolekar et al. have reported that nitration of toluene using nitrogen dioxide was possible on zeolite catalysts such as H-ZSM-5, H-modernite, and HY. (see Akolekar D. B., et al., Res. Chem. Interm. 21, 7 (1995)). However, total conversion was not over 60% even though excess nitrogen dioxide (8-10 equivalent) was used under very high pressure (2,430 psi) and temperature (110.degree. C.). Moreover, Sato et al. have disclosed vapor-phase processes for the nitration of benzene with nitrogen dioxide or nitric acid in the presence of catalyst of acidic mixed metal oxides. (see U.S. Pat. No. 4,551,568 (1985), U.S. Pat. No. 5,004,846 (1990), U.S. Pat. No. 5,030,776 (1991)).
The inventors of the present invention have developed a nitration process using nitrogen dioxide and an inorganic salt type catalyst (Korean Patent Application No. 97-27617). According to the process, nitrogen dioxide is activated and aromatic compounds are effectively nitrated by pressurized oxygen without using an additional solvent. However, in this process, since an inorganic salt type catalyst such as ferric chloride can be converted into an organometal type catalyst by reacting with nitrogen dioxide and nitric acid produced during the process, and the solubility of the catalyst in the reaction mixture increases, it is difficult to recover the catalysts by simple filtration after the reaction. Thus, significant amount of the catalyst dissolved in the reaction mixture must be removed by washing with water.